Wikipedia defines an endplay in bridge as “a tactical play where a defender is put on lead at a strategic moment, and then has to make a play that loses one or more tricks. Most commonly the losing play either constitutes a free finesse, or else it gives declarer a ruff and discard.”

Here’s an example roughly based on an online deal. South is dealing, with no one vulnerable. Here are the North-South hands:
      

NORTH                    
♠️ A K 9 4
♥️ K 9 7                    
♦️ J 8                 
♣️ A 8 6 4   

SOUTH
♠️ Q J 10 7                   
♥️ A J 8                    
♦️ A 5
♣️ Q J 10 3
            
The bidding proceeded as follows:
South               West           North          East
1NT                  Pass            2♣️             Pass
2♠️                  Pass            4♣️             Pass
4NT                 Pass            6♠️            All Pass     

Roman Keycard Gerber (4NT would be a quantitative bid denying a spade fit and asking partner to bid 6NT with something better than a bare-bones 1NT opening bid).

Two of the five key cards (four aces, trump king), plus the ♠️Q

Opening lead: ♦️K

South won with the ace. After drawing trump, ending in hand, South played four rounds of clubs, West wisely holding off as long as possible before playing the king. Here’s how things stood:

NORTH                    
♠️ 4
♥️ K 9 7
♦️ J
♣️  —

SOUTH
♠️ J
♥️ A J 8                    
♦️ 5
♣️ —

At this point, most declarers would play the ♥️K and finesse in that suit. Except that the finesse fails, the ♦️Q is cashed, and North-South go down one. But there’s a way of making the contract without finessing in hearts. Throw West in with a diamond!

If West leads hearts, there’s no way the defenders can claim a trick in the suit. If West plays a diamond, declarer can ruff while discarding a heart. Any way you look at it, East-West are endplayed.


The hands:
                                   

                                      NORTH                    
                                     ♠️ A K 9 4
                                     ♥️ K 9 7                    
                                     ♦️ J 8                 
                                     ♣️ A 8 6 4 

WEST                                                         EAST
♠️  3 2                                                      ♠️ 8 6 5
♥️ 10 6 4                                                  ♥️ Q 5 3 2
♦️ K Q 10 9                                             ♦️ 7 6 4 3 2
♣️ K 7 5 2                                                ♣️ 9 


                                   SOUTH
                                  ♠️ Q J 10 7                  
                                  ♥️ A J 8                    
                                  ♦️ A 5
                                  ♣️ Q J 10 3